The present invention relates to nickel cathodes. More particularly, this invention relates to methods of treating nickel cathodes to prevent pyrophoricity.
Alkali metal hydroxide and chlorine are commercially produced by electrolyzing an alkali metal chloride brine, for example, an aqueous solution of sodium chloride or an aqueous solution of potassium chloride. The alkali metal chloride solution is fed into the anolyte compartment of an electrolytic cell, a voltage is imposed across the cell, chlorine is evolved at the anode, alkali metal hydroxide is evolved in the catholyte, and hydrogen is evolved at the cathode.
While iron or mild steel has often been used as the cathode in conventional commercial chlorine cells, it has been found that a reduction in hydrogen overvoltage may be realized by utilizing a cathode having a porous high surface area coating, e.g., porous Raney nickel. Use of such a porous Raney nickel cathode coating can produce a significant savings in energy consumption and cost.
Generally, a porous Raney nickel coated electrode is formed by depositing a nickel-aluminum alloy layer upon a metal substrate followed by a selective leaching step wherein sufficient aluminum is removed to form an active high surface area nickel layer. A strong aqueous base, such as sodium hydroxide or potassium hydroxide, capable of dissolving aluminum is used in the selective leaching step. After selective leaching, active nickel coatings exhibit a tendency to heat upon exposure to air. This self-heating tendency can lead to problems of pyrophoricity. Self-heating pyrophoric cathodes can be hazardous to personnel working with such electrolytic cells. Further, such heating can cause damage to the cathode substrate itself or in an electrolytic membrane cell cause damage to the ion exchange membrane.
A number of methods have been developed to lessen the sensitivity of porous Raney nickel coated electrodes to air. It has been suggested to treat such electrodes with a dilute aqueous solution containing a chemical oxidizing agent, for example, solutions containing by weight, (a) 3 percent sodium hypochlorite and 10 percent sodium hydroxide, (b) 3 percent sodium nitrate, or (c) 3 percent potassium dichromate. Treatment with a dilute solution of hydrogen peroxide has also been proposed to reduce the pyrophoric tendency of such an electrode.